Due to the high demand for iron oxides that has existed over many decades, sources of high grade ore have become depleted or, in some areas, exhausted. As a result, a great deal of attention has been given to the development of technology to recover iron oxides from low grade iron ore materials. As used herein, the term “low grade iron ore” refers to a material that is composed of a mixture of one or more iron oxide and substantial amounts of one or more non-iron impurity, commonly one or more of quartz, chert, carbonate or the like. One example of a low grade iron ore material is the iron ore commonly referred to as taconite, an iron-bearing sedimentary rock, typically having an iron oxide content of from about 15% to about 40%, with the balance being non-iron impurities. For purposes of the present disclosure, the term “taconite” is used to refer to any natural iron ore material that is composed of a mixture of one or more iron oxides and substantial amounts of one or more non-iron impurities.
Because the iron oxide material in many taconite formations is at least partially in the form of magnetite, which is a mineral strongly influenced by a magnetic field, technology was developed many decades ago that utilized low intensity magnetic fields to separate magnetite and other minerals strongly influenced by a magnetic field, such as, for example, martite, and maghemite, from the other materials in taconite ores. The term “low intensity magnetic separator” refers to a separator that separates highly magnetically susceptible particles such as magnetite particles from particles that are weakly susceptible or non-susceptible to a magnetic field. Low intensity magnetic separators effect separation by subjecting a stream of mixed particles to a relatively low magnetic field having a strength. A wide variety of such separators, some of which are structured to separate a feed stream of dry particles and others structured to separate a feed stream of particles suspended in water (i.e., slurries), are described in the prior art. Low intensity magnetic separators are effective to separate the taconite ore into a magnetite concentrate fraction, which includes a higher concentration of magnetite than the starting taconite ore, and a tailings fraction, which includes a higher concentration of the non-magnetite materials than the starting particulate taconite ore.
Taconite ores, in addition to including magnetite, typically also include substantial amounts of iron oxides in the form of hematite or other iron oxides that are only weakly influenced by magnetic fields. In a low intensity magnetic separator, these non-magnetite iron oxides pass into the tailings fraction of the low intensity magnetic separation operation together with non-iron impurities. A substantial quantity of taconite tailings from prior low intensity magnetic separation operations have been placed in reject tailings deposition basins through the years. Other tailings materials that also include usable quantities of iron oxides include, for example, iron oxide tailings from natural iron ore wash, density separation, sluicing plants, or heavy media processing plants. In addition, other taconite ores and other iron-bearing ores have been mined and ground to particulate form, but subsequently deemed unsuitable for further processing with low intensity magnetic separators due to a determination that they include insufficient quantities of magnetite to make the operation economically feasible. Many tons of such materials have been placed in lean ore stockpiles. These tailings basins and stockpiles represent a collection of elements in a form that already has considerable energy, manpower and “carbon footprint” invested into the mining and size reduction of the rock involved and therefore such occurrences have even greater economic and environmental appeal in view of concerns regarding pollution and climate change. For purposes of the present disclosure, such tailings and stockpiled lean ores (whether referred to as taconite ores or by another name), together with lean ores in their natural state (i.e., unmined and/or unground), whether or not they include some amount of magnetite, and whether they include hematite, iron oxides other than hematite, or both, are referred to herein as “low grade mineral assemblages.”
Economically feasible extraction of iron oxides from low grade mineral assemblages, whether present in their natural state, in lean or stockpiles or in tailings of prior mining or mineral processing operations, requires the use of energy efficient processes effective to separate the low grade mineral assemblages into a particulate fraction that includes iron oxides having iron concentrations that are sufficiently increased to have commercial value (referred to herein as a “concentrate”). The separation process can be very simple, involving few unit processes, or very complex, involving many unit processes. Substantial attention has been given over many decades to the development of processes for producing a concentrate from low grade mineral assemblages. Generally, such processes involve one or more unit processes within the general categories of comminution, separation and dewatering.
Comminution (also referred to as size reduction) typically involves crushing, followed by grinding, to reduce the size of the ore to a point that the minerals are liberated from one another and to prepare the material for physical and/or chemical separation. Comminution by crushing and/or grinding can be accomplished using, for example, a cone crushing device, a jaw crushing device, a roll press, a rod mill, a ball mill or a tower mill, each of which is well known in the relevant field and commercially available. In the case of tailings and lean ore stockpiles, the low grade ore materials have already been comminuted to a certain degree; however, further comminution may be required in some cases to optimize iron oxide recovery.
Once the ore has been sized, the mineral assemblages are then separated into fractions by one or more of the following unit processes: size separation, gravity separation, electrical or magnetic separation and froth flotation. Size separation uses the difference in particle size of the different minerals (e.g., washing clay from sand on a screen). Gravity separation uses the difference in density or specific gravity of the minerals. Equipment commonly used for gravity separation includes dense or heavy media, shaking tables, spirals, barrel washers, or jigs. Electrical or magnetic separation uses those respective physical properties of the minerals to effect separation. Froth flotation uses surface chemistry differences in the minerals. As will be appreciated by a person of ordinary skill in the art, the term “separated” as used herein is not intended to require complete separation of iron oxides from gangue materials, but rather refers to the separation of the low grade ore material into a fraction having a higher concentration of iron oxides/lower concentration of gangue materials (referred to herein as a “concentrate fraction”) and a fraction having a lower concentration of iron oxides/higher concentration of gangue materials (referred to herein as a “tailings fraction”). An object of all large-scale separation processes is to optimize the efficiency, productivity and profitability of the separation process by balancing the degree of separation of iron oxides from non-iron materials present in a mineral assemblage with the cost of each incremental increase in the degree of separation.
To use the above separation unit processes, a mineral assemblage typically must first be put into a slurry form. The term “slurry” is used herein to refer to a fluid-mineral suspension of the mineral assemblage in which the mineral particles are suspended in liquid water. A mineral assemblage can be provided as a slurry by mixing the mineral assemblage with water either during or subsequent to mining excavation of the mineral assemblage. Because the above separation unit processes require the suspension of the low grade mineral assemblage in water to form a slurry prior to the separation treatment, the resulting concentrate fraction slurry and tailings fraction slurry, respectively, need to be dewatered so they can be transported (in the case of concentrate) or disposed of in an environmentally acceptable manner (in the case of tailings). Examples of dewatering devices that can be used for this purpose include deslimers, hydro-cyclones, spiral classifiers, thickeners, clarifiers, vacuum filters, pressure filters, multi-roll filters, centrifuges and elutriator sumps. A variety of suitable dewatering devices are known in the art and are available commercially, and it is well within the purview of a skilled artisan in view of the present descriptions to select, obtain and use a suitable dewatering device in the methods described herein.
Substantial efforts have been made to develop processes for producing iron oxide concentrates from low grade mineral assemblages. Until recently, however, such processes had not proven to be sufficiently effective, or sufficiently economically feasible, for use on a large scale. Recent developments in magnetic separation technology, however, have provided breakthroughs that enable extraction of large volumes of weakly magnetically susceptible iron oxides from low grade mineral assemblages. In these processes, a particulate mineral assemblage is suspended in water and the resulting aqueous suspension is passed through a magnetic separator that produces a relatively high intensity magnetic field, to separate the mineral assemblage into a concentrate fraction having a higher iron oxide concentration than the mineral assemblage and a tailings fraction having a lower iron oxide concentration than the mineral assemblage. Due to the relatively strong magnetic field that is necessary to influence the trajectories of iron oxides that are only weakly susceptible to magnetic fields, and the need to suspend the low grade mineral assemblages in water to form a slurry before passage through the magnetic field, devices that are used in this type of process have come to be referred to as wet high-intensity magnetic separation devices, or WHIMS devices.
One type of WHIMS device includes flux amplifying matrix materials to provide points of high magnetic attraction within a flow path through which a mineral assemblage slurry is passed. Such devices are a particular subset of the more general WHIMS category of magnetic separators, and are referred to herein as WHIMS-FAM devices (Wet High Intensity Magnetic Separation using Flux Amplifying Matrix). A WHIMS-FAM device defines at least one flow path (and typically several flow paths) for passage of aqueous slurries of the mineral assemblages in particulate form, includes sources of at least one (and typically several) high intensity magnetic field whose flux lines pass through the flow path, and include flux amplifying matrix materials contained within the flow path. The flux amplifying matrix can be, for example, iron or steel shot, steel rods, steel wool, steel parallel plates, wire mesh, machined iron or steel plates, V-shaped steel parallel plates, iron or steel hex nuts, or other discrete iron or steel pieces or shapes. The flux amplifying matrix operates in a high intensity magnetic separation device by concentrating flux lines between magnet poles so as to produce localized points of very high magnetic attraction within the slurry flow path, which attract faintly magnetic particles to separate the faintly magnetic particles from non-magnetic mineral particles. Using discrete object flux amplifying matrix materials in a high intensity magnetic separator, a higher concentration of flux lines can be achieved, producing higher localized magnetic field strengths at the contact points between the discrete objects when present in a magnetic zone.
By intermittently passing the matrix materials into a magnetic field where a low grade mineral assemblage treatment slurry is passed in contact with the flux amplifying matrix and iron oxide particles are held in contact with the matrix, and then out of the magnetic field to flush the iron oxide particles from the matrix, the mineral assemblage slurry can be separated into a concentrate fraction and a tailings fraction. Examples of WHIMS-FAM devices are those described in U.S. Pat. No. 7,886,913, issued Feb. 15, 2011, U.S. Pat. No. 8,292,084, issued Oct. 23, 2012, and U.S. patent application Ser. No. 13/452,420, filed Apr. 20, 2012, each of which is incorporated herein by reference in its entirety.
One challenge that has been encountered in the use of WHIMS-FAM devices is that they tend to become clogged during normal operation. For example, debris and organic matter such as leaves and vegetation, and/or oversize particles in the treatment slurry and/or magnetite or other highly magnetically susceptible particles or objects can become lodged in the matrix, and then additional particles can build up thereon irrespective of their magnetic susceptibilities. This particle build up can significantly impair the flow of a treatment slurry through the flow path, resulting in substantial productivity losses. One manner in which the clogging problem has been addressed is to use pre-separation slurry preparation steps to carefully control the particle sizes in the treatment slurries to be introduced into such devices, to carefully remove debris and organic matter such as leaves and vegetation, and/or oversize particles from the treatment slurry and to remove magnetite particles from the treatment slurry. Another approach for addressing the clogging problem has been to implement back-flushing steps in the separation process and/or to develop separator devices in which matrix material components and/or other separator components that contain matrix materials can be quickly replaced without causing unacceptable delays and shut-down times for the separators.
Another challenge associated with the use of WHIMS-FAM devices is that there is a practical limitation to the volume of slurry that can be treated in a given period of time. One cause of this limitation is the nature of the slurry flow paths in such a separator. Because the flow paths contain relatively high surface area flux amplifying matrix materials and because thorough separation of weakly magnetic particles in the mineral assemblage from non-magnetic materials requires thorough contact between the treatment slurry and the matrix materials, a substantial amount of turbulence and mixing of the slurry occurs in the flow path. As a result, the volumetric rate at which slurry can pass through the WHIMS-FAM device is limited.
Yet another challenge associated with iron ore upgrading processes utilizing WHIMS-FAM separator devices is that the processes required to provide a treatment slurry in a form suitable to be passed through a WHIMS-FAM device require the addition of a substantial amount of process water to the slurry. For example, when passing mineral assemblage slurries through screening devices and other size separators, a significant amount of process water is added to agitate the feed material, to rinse undersize particles through the screen and to remove oversize particles from the screen to an oversize fraction. As a result, in order to efficiently operate the separator, a substantial amount of excess water must subsequently be removed from the treatment slurry prior to introduction of the treatment slurry into a WHIMS-FAM separator.
Excess water can be removed from a slurry by passing the slurry through a dewatering device, such as, for example, a deslimer, hydrocyclone, thickener, hydroseparator or elutriator sump. The overflow water from the dewatering device(s) can be conveyed to a reservoir or a settling pond and can optionally be recycled for further use in the process as process water. However, because low grade mineral assemblages often include fine or ultrafine particles (e.g., particles having dimensions of less than about 100 microns), these dewatering treatments can result in substantial losses of iron oxide materials in the overflow streams of the dewatering device, and associated loss of productivity of the process. The dewatering step(s) theoretically could be omitted to prevent this loss of ultrafine particles; however, eliminating the dewatering step(s) would also result in unacceptable productivity losses due to the lower solids to water ratio that would result and the limitations on volumetric flow rates through a WHIMS-FAM device, as discussed above.
In addition, process water is also used within a WHIMS-FAM separator to rinse and flush collected iron oxide particles from the flux amplifying matrix into a concentrate fraction after it moves out of a magnetic zone. Therefore, an iron oxide concentrate fraction recovered from a WHIMS-FAM device also must be dewatered to produce a final concentrate product. The concentrate fraction slurry also can be dewatered by passage of the slurry through a dewatering device, such as, for example, a deslimer, hydrocyclone, thickener, hydroseparator or elutriator sump prior to filtering and/or other final drying operations, and the overflow water from such a dewatering device can also be conveyed to a reservoir or settling pond and optionally can be recycled for further use in the process as process water. Dewatering operations used to dewater the final concentrate fraction recovered from a WHIMS-FAM device, however, are also susceptible to the same problems discussed above, i.e., the unintentional loss of fine or ultrafine iron ore particles into an overflow stream removed from the dewatering device(s), which also can result in substantial losses of iron oxide materials, and associated loss of productivity of the process.
There is an ongoing need, therefore, for advancements relating to the recovery of iron oxide from low grade mineral assemblages and, in particular, advancements in the productivity of WHIMS-FAM devices. The present disclosure addresses this need.